Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly, to an image pickup apparatus to which an interchangeable lens is mountable, and which has a function of changing a focal length of the interchangeable lens, and to an image pickup system including the same.
Description of the Related Art
Hitherto, there have been proposed various methods of arranging an optical system between a photographing optical system (main lens system) and an image pickup element in order to change a focal length of the photographing optical system. For example, in Japanese Patent Application Laid-Open No. 2000-19613, there is proposed an image pickup apparatus, including an optical system configured to change a focal length of a photographing optical system to increase the focal length. In addition, in each of Japanese Patent Application Laid-Open No. H11-258499 and Japanese Patent Application Laid-Open No. H11-194268, there is proposed an attachment capable of increasing a focal length of an interchangeable lens serving as a photographing optical system by inserting an optical system between the interchangeable lens and an image pickup apparatus.
In recent years, image pickup apparatus, such as a television camera, a movie camera, a photographic camera, and a video camera, have been desired to increase both the number of pixels and sensitivity. Thus, there has been demanded an image pickup apparatus including a large-sized image pickup element. At the same time, a user may wish to use a resource of an existing interchangeable lens. Accordingly, for example, there is a strong need to use a ⅔-inch format interchangeable lens for a super-35 mm format image pickup apparatus including a larger image pickup element. In this case, an optical system configured to increase the focal length of the interchangeable lens needs to be arranged between the interchangeable lens and the image pickup element to enlarge an image circle of the interchangeable lens.
In Japanese Patent Application Laid-Open No. 2000-19613, there is proposed an image pickup apparatus according to one embodiment, in which a photographing optical system (main lens system) is arranged integrally with an image pickup apparatus main body. Thus, an interchangeable lens cannot be mounted to this image pickup apparatus. In Japanese Patent Application Laid-Open No. H11-258499 and Japanese Patent Application Laid-Open No. H11-194268, reduction in size and securement of a backfocus are not achieved sufficiently at the same time. Thus, further improvement is required.